Photochemical Bergman

This analysis confirms that the effect of cyclic constraints is not purely steric but also has an electronic component. Another aspect of this dichotomy is shown in Fig. 11 which illustrates the decrease in the energy gap between the frontier in-plane rc-MOs. The decrease in the C1-C6 distance destabilizes the occupied MO where the interaction between the end orbitals is antibonding and, at the same time, stabilizes the empty MO where the 7i -orbitals overlap constructively. As a result, the efficiency of the photochemical Bergman cyclization should increase and, indeed, the most efficient photo-Bergman cyclizations reported in the literature involve cyclic enediynes.43 Again, the analogy with interrupted [2 + 2] photocycloaddition is instructive. 

Strain increase effect on the photochemical Bergman cyclization... 

A very interesting experimental observation of Jones et al. illustrates a different effect of strain on the efficiency of photochemical Bergman cyclizations.43d Variations in the size of the cycle which does not incorporate the whole enediyne system, but only the vinyl part of the enediyne moiety (in contrast to the previously discussed data) affect the yield of the cycloaromatized product. Initially, an increase in the ring size leads to an increase in yield (Scheme 16). 

Scheme 16 Model enediynes used in photochemical Bergman cyclization.

Clark, A. E., Davidson, E. R., Zaleski, J. M. UDFT and MCSCF Descriptions of the Photochemical Bergman Cyclization of Enediynes. J. 

Electronic Structure Contributions to Thermal Bergman CycUzation / 445 Photochemical Bergman CycUzation of Metalloenediynes / 449 Porphyrin—Enediyne Conjugates / 453... 

SCHEME 30.1 First report of photochemical Bergman cyclization. 

SCHEME 30.2 Photochemical Bergman cyclization of aromatic enediynes. 

Branda and coworkers provided another example of an acyclic enediyne that undergoes a photochemical reaction different from a photo-Bergman ring closure [21 ]. In this case, photochemical Bergman in strained enediynes is slower than photochromic bx-electrocychc ring closures. Only the latter process is observed upon UV (365 nm) excitation of the strained enediyne shown in Scheme 30.8. Branda and coworkers used the reversibility of this process for an on-demand uncaging of thermally reactive enediynes by visible light (>525 nm). 

In addition to the aforementioned transformations, the photo-induced Bergman cyclization was utilized by Zhu and coworkers to form intramolecular polymeric nanoparticles.The appropriate reactive diyne unit was incorporated into the polymer using both direct controlled radical polymerization and aHqme protection strategies. The resulting linear polymers were subjected to photochemical Bergman cyclization conditions using a continuous addition strategy, similar to the case previously discussed. ... 

Based partly on the success of benzofused enediynes such as 28, 30, 31, and 34, Wiest prepared highly strained enediynes 36 and 37 and subjected them to conditions suitable for photochemical Bergman cyclization. These molecules proved stable to prolonged high-intensity irradiation. Based on calculations, it was suggested that the unexpected photochemical (and thermal) stability of 36 and 37 is the result of diyl intermediates that are unusually high in energy. 

Alternatively, conversion to bromides 45 followed by carbenoid coupling gave 46 (n = 1-5) in good yield on a preparative scale. Though stable at room temperature, photochemical Bergman cycloaromatization gave adducts 48 in moderate yield, presumably via diyls 48 (Table 29.3). ... 

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